In the field of light scattering, as applied to determine the molar mass and mean square radius of solvated molecules, measurements are made from solutions comprised of a solvent containing a dissolved sample. By measuring the scattered light variation with scattering angle and measuring the concentration of the solute, one may in principle determine the molar mass and mean square radius of such solvated molecules. Similarly, the light scattering properties of sub micrometer particles in liquid suspension may be used to determine their average size Light scattering techniques may be applied as well for measurements involving inelastic light scattering such as photon correlation spectroscopy, Raman spectroscopy, fluorescence, etc. These measurements, usually performed at a fixed single angle, are used to determine the hydrodynamic size of the particles or molecules illuminated.
Light scattering measurements are often made with a light scattering photometer wherein the sample is introduced into an optical cell such as referenced above in U.S. Pat. Nos. 4,616,927 and 5,404,217. Interfering with such optical measurements are a variety of contaminants whose presence inside the flow cell often contribute to the recorded light scattering signals in such a manner as to distort or even mask them Such contaminants arise from various sources, many of which cannot be avoided. Included among these are small air bubbles, fine particles shedding from chromatographic columns if such are being employed to separate the molecules or sub-micrometer particles nor to measurement, aggregates formed from the sample itself which may have a strong affinity for the internal optical surfaces, contaminants in the poorly prepared solvent, debris from previous measurements that build up on the optical surfaces, etc.
During the measurement process, the presence of these contaminants are often recognized indirectly through the effects they have on the scattering or are noticeably visible through physical examination of the scattering cell, or both. There are various means by which such contaminants are removed or dislodged from the internal optical surfaces such as flushing the optical cell with different solvents such as acids or detergents or introducing a large air bubble in the manner of the familiar Technicon AutoAnalyzer of the 1960s. Sometimes, no matter how much effort has been expended, the flow cell must be disassembled and each component cleaned manually. Once disassembled, one of the most useful means for cleaning surfaces is to use ultrasonic waves as created in an ultrasonic cleaning bath. The components are placed in a fluid such as water and ultrasonic waves whose fixed frequencies are of the order 50 kHz are propagated throughout the bath These waves are generally generated by means of piezoelectric transducers well coupled to the bath chamber. At the frequencies and power levels traditionally applied, cavitation effects generally cause the generation of bubbles which, when driven against a surface, tend to assist in the cleaning and scrubbing of such surfaces.
Although the disassembly of an optical cell and the subsequent cleaning of its parts in an ultrasonic bath are effective, it is time consuming. Unfortunately, it is often the only means possible. When the optical cell is used in a high temperature environment, such as is the case for chromatographic separations requiring high temperature solvents, the traditional disassembly concept becomes even more time-consuming since the temperature of the chromatograph itself must often be reduced significantly to obtain access to the optical cell, which is then removed and cleaned. High temperature chromatographs, and especially the columns used therein, can be damaged during temperature cycling, which, therefore, must be carefully executed. The process of cleaning an internally mounted optical cell can, in such a case, require up to 24 hours to effect a removal, cleaning, and reinstallation.
It always has been thought desirable to have optical elements of the light scattering cells designed in such a manner as to prevent the deposition of extraneous materials on their surfaces or, at the very least, design them in such a manner as to permit the cleaning of their internal surfaces with minimal effort. To this end, many structures requiring clean, particulate-free surfaces have been designated as xe2x80x9cself-cleaningxe2x80x9d such that once internal precipitants are detected they may be removed without need for disassembling the structures themselves. A process by which the initial formation of such contaminants may be reduced is taught, for example, by Davidson in U.S. Pat. No. 5,442,437 wherein windows, through which optical measurements are to be made, are so positioned that they extend into the flowing solution which, thereby, continuously xe2x80x9c. . . scour said window to minimize contamination and clouding [thereof] . . . xe2x80x9d This, of course, is an old concept that was disclosed in U.S. Pat. No. 4,616,927, referenced above, and numerous other similar implementations whereby it is necessary to clean observation windows of various types. Although such cleaning may keep the observation windows clear of particulate debris for some time, eventually sufficient particles may accrete so as to interfere with light passing through some optical surface.
Another example of a self cleaning cell is U.S. Pat. No. 4,874,243 by Perren wherein the windows are at an angle to the direction of flow which results in a xe2x80x9c. . . self cleaning action . . . xe2x80x9d as the flowing stream passes over them. A similar example is U.S. Pat. No. 4,330,206 of Gausmann et al. wherein is shown a measurement chamber xe2x80x9c. . . inherently self-clearing of air or gas bubbles in liquid samples . . . [which provide] inherently efficient cleansing of the measurement chamber . . . xe2x80x9d This is achieved by outlet means lying above the optical region guiding thereby air bubbles up and out of the fluid enclosed. The fluid flowing into the measurement channel strikes the cell window obliquely, thus cleaning it and maintaining it free of contaminants.
Berger in his U.S. Pat. No. 4,496,454 describes another example of a self-cleaning mechanism for the case of electrochemical cells used with certain forms of liquid chromatography. His invention attacks a similar problem for electrochemical detection that faces light scattering detection: the fouling of the electrode surfaces during measurement which, in turn, affects the detector response. In the light scattering case, the optical surfaces can become fouled with particulates and small air bubbles. Berger achieves his cleaning by using a capillary tube to generate a water jet perpendicular to the detector electrode surface
In addition to such fluid cleaning means described above, there exist a number of mechanical means exemplified by Wynn in his U.S. Pat. No. 5,185,531. In Wynn""s implementation, the optical windows are kept clean by introducing periodically mechanically controlled flexible wiper blades xe2x80x9c. . . extending from opposite sides of [a] . . . blade holder for wiping engagement with the window surfaces . . . xe2x80x9d Other implementations of a wiping motion to clean an optical cell may be found in U.S. Pat. No. 3,844,661 by Birkett et al. or U.S. Pat. No. 4,074,217 by Yanagawa.
Although the use of ultrasonic waves appears an attractive means for removing particulates from surfaces, such as described by Neefe in his U.S. Pat. No. 4,457,880, it has never been used as a component of an optical cell to permit self cleaning action. There are three basic reasons for this omission. First is the fact that there has been neither means for establishing a proper frequency regime to achieve such cleaning nor means for localizing the cleaning action to the internal cell surfaces that require it. Secondly, even were such a self cleaning device integrated with the cell structure, there could be no assurance that, once removed from the internal cell surfaces, the particulates would not re-adhere or simply remain within the cell to adhere later to some other region. Finally, traditional ultrasonic waves used in cleaning are generated at frequencies of the order or 50 kHz, which, at the power levels traditionally employed and in fluids such as water, induce cavitation effects that result in the generation of bubbles. Such bubbles are most helpful because of their implied scrubbing action on the surfaces to be cleaned. Were such bubbles generated within an optical cell, the bubbles themselves could be expected to adhere to surfaces within the fine interstices of such cells defeating, thereby, the cleaning concept ab initio.
Ohhashi in his U.S. Pat. No. 4,672,984 extends the ultrasonic concept for cleaning optical surfaces by providing a plurality of cleaning steps, each of which may involve a different working liquid and/or ultrasonic intensity applied over varying periods of time. Once again, such cleaning is done externally to any enclosed structure with the parts to be cleaned transported individually to the array of cleaning baths. He does not discuss the frequency of the applied ultrasonic frequencies nor any possible variations thereof, so one assumes that he employs the standard cavitation prone frequencies around 50 kHz,
Honda et al. in their U.S. Pat. No. 5,656,095 introduce the concept of multiple frequencies, some of which are applied intermittently to destroy the bubbles generated by the continuously applied frequency. Such an action results in corresponding pressure pulses to which is attributed a xe2x80x9c. . . greatly improved . . .xe2x80x9d washing effect. They consider so-called low frequency generation as occurring at frequencies of 28 kHz, 45 kHz, and 100 kHz whereas high frequency generation describes generation at 160 kHz. The high frequency ultrasonic waves are said to generate bubbles in the size range of 20 xcexcm to 500 xcexcm while the intermittent low frequency waves destroy the bubbles, generating as they collapse, even higher orders of ultrasonic waves.
The present invention is concerned with the implementation of an ultrasonic cleaning device that is integrated with an optical flow cell and controlled in such a manner as to permit sonic coupling with those internal regions of the cell most needed to be particulate free. Sonic waves are used in a manner by which cavitation is avoided whenever possible since such cavitation can cause etching or other damage to finely polished optical surfaces.
This invention presents a new design concept for the cleaning of optical surfaces within flow cells used in conjunction with light scattering measurements such as commonly employed in the field of analytical chemistry and, more particularly, for liquid chromatography. Basic to this invention is the incorporation into the flow cell structure itself of means to provide internal to the flow cell extremely high frequency sonic waves such as would be produced by means of an electrically driven piezoelectric transducer. The frequencies of these waves are much greater than those employed by Honda et al. In order to avoid cavitation, yet be in resonance with the typical internal dimensions of the cleansed flow cells, frequencies of the order of 1 MHz/sec are employed. There are many different types of flow cells for which this design would be useful including those referenced above. In B. Chu""s textbook on xe2x80x9cLaser light scatteringxe2x80x9d, a number of additional designs may be found; though these are by no means exhaustive.
Key to this invention are four features: 1) integrating, by good mechanical contact means, the sonic source, a piezoelectric transducer in the preferred embodiment, and the optical flow cell; 2) varying the frequency of the applied ultrasonic waves so as to couple well with those internal regions where the dislodgment of particulates is required; 3) using frequencies of the order of a MHz which are much greater than those traditionally used for ultrasonic cleaning purposes and, at practical power levels, beyond the frequencies that conventionally would cause cavitation in most liquids; and 4) providing a flowing fluid means during the application of the ultrasonic waves by which dislodged particulates may be removed from the cell.
Although such an integrated cleaning technique may be applied to static optical cells that are not generally operated in a flow through mode, when used with such mechanically coupled ultrasonic waves, means must be provided to permit a flow stream to remove particles dislodged by said sonic cleaning during the application of said ultrasonic waves.
The requirement that the frequency of the applied sonic waves must be adjustable, so as to couple the sonic energy most efficiently to the internal regions of the flow cell structure most prone to the presence of unwanted particulates, may equally well be served by automatically, and repetitively, scanning a range of frequencies that includes those best suited for the internal regions to be cleaned. Note that at frequencies of the order of 1 MHz in water, the associated wavelengths are of the order of 1.5 mm, approximately the diameter of the flow cell of the flow cell of U.S. Pat. Nos. 4,616,927 and 5,404,217 and related structures. The dislodgment of particles by the present inventive means relies upon mechanical displacement by the ultrasonic waves themselves rather than the more traditional scrubbing action created in large measure by the cavitation created air bubbles turbulently bombarding the affected surfaces.
The fluid that must be flowing through the flow cell structure during application of the ultrasonic waves throughout the structure must be in itself particle-free. When applied to a flow cell in conjunction with a chromatographic separation, this fluid would correspond to the so-called mobile phase of the chromatographic separation process. Such fluids should be free of particulates and are often degaussed and filtered prior to use in the chromatograph. Additionally, since the ultrasonic field can induce particle aggregation within the flow cell, the resulting aggregates are more easily flushed from the flow cell.